corrected mistake

binarycpython/utils/grid.py

@@ -618,8 +618,8 @@ class Population(object):
         print("with mp: {} systems took {}s".format(amt, stop_mp - start_mp))
 
         #########################################################
-        duration_no_mp = start_no_mp - stop_no_mp
-        duration_mp = start_mp - stop_mp
+        duration_no_mp = stop_no_mp - start_no_mp
+        duration_mp = stop_mp - start_mp 
         ratio = duration_no_mp/duration_mp
         print(
             "Running mp versus no mp is {} times faster!".format(